1. Technical Field
The present invention relates in general to improved processing for ion beam etching and, in particular, to an improved system, method, and apparatus for improving the process stability of ion beam etching by using a step reference for time scaling of subsequent steps of the process.
2. Description of the Related Art
Ion milling is used in integrated circuit and thin film component fabrication to remove coatings either partially or completely from the component at various steps of the fabrication process. A coating is removed from a substrate, such as a semiconductor wafer, by being placed in a vacuum chamber in which a gas is ionized to generate reactive ions. The removal of the coating is achieved by a chemical reaction between the ions and the coating, as well as by bombardment of the coating by the ions. The gaseous reaction products are continuously removed from the chamber by a vacuum pump. The terms ion milling, ion etching, and plasma etching all relate to the same general process and may be used interchangeably. However, in some applications, there is no chemical reaction involved in the process when Nobel gases are used for ion mill.
Ion milling applications are subject to run-to-run process variation due to several factors. For example, tool drift or workpiece drift (e.g., from wafer to wafer) due to heating, deformation, material density, layer resistance, charging, thickness, etc., can cause variations. The approach currently used to solve such drift problems is to adjust a suitable process variable based on observations and measurements from previous runs (i.e., trial and error). For example, a secondary ion mass spectrometer end point detector (SIMS EPD) may be used to control the milling depth in some applications. It can be used to start milling or stop milling when a pre-determined layer has been reached. The duration depends solely on the milling rate, which fluctuates based on the conditions of the tool when a sample is processing.
As shown in FIG. 1, an ion mill process includes a workpiece or substrate 21, such as alumina, a sensor structure 23, a mask 25, and an ion beam 27. During the ion mill process, the time durations of the first step 11 and the third step 15 are controlled by a SIMS EPD. The time durations of the second step 13 and fourth step 17 are calculated using the mill rate (i.e., E_rate). The mill rate is only monitored periodically, typically about every 48 hours. Each time the mill rate changes, the time durations for the second and fourth steps 13, 17 are corrected accordingly.
One problem with this approach is the delay (i.e., as much as 48 hours) between the time that the measurements are taken and the actual implementation of the changes to the tools. During this delay, the tool conditions continue to drift such that, by the time the changes are made, additional changes are needed to compensate for the additional drift that has been incurred during the time delay. Thus, an improved and more timely system for compensating for drift would be very desirable.